Re: asm on san

On 18 Dec, 15:02, DA Morgan <damor..._at_psoug.org> wrote:
> Ralph.in..._at_googlemail.com wrote:> > Hi All,>> > My experiances in the last 8 yrs of the Enterprise storage> > environment do not bear out the anti RAID 5 people. RAID 5 is used> > extensively in this financial services environment.>> Which translates to the fact that storage vendors oversold hardware.> Anyone can go fast if they buy a Ferrari.>> The relevant question is not what did the now disgraced financial> services firms due squandering their money (thus they are now being> bailed out by taxpayers) but rather how responsible companies,> spending their money wisely, can get adequate performance for> reasonable dollars.

Or you can look at it another way... by using Raid 5, which has been
proven sufficient, rather than saying "its a database - it must be
RAID 10" They have saved a fortune on disk spindles and the power,
cooling etc by putting in an adequate solution rather than an
overblown one.

>> You can not write the same byte 3 times and get performance identical> with a system that only write them twice any more than you can drive> you car three miles on two miles worth of petrol. Physics doesn't> subscribe to marketing hyperbole and DBAs shouldn't either.

Clearly not, but you are oversimplifying the issue to prove your
point. You need to look holistically at the storage device and the
actual cost of writing the data and the parity information. The data
in the cache that needs to be written to the spindles is written in an
intelligent manner to minimise head movement etc., even though the
same physical disk may actually participate in many actual luns that
are presnted to any number of different systems. Now clearly without
this super big cache and the intelligent writing mechanisms this would
never work...however it works like a dream. So while you are right
that 3 writes will take longer than 2, in reality this doesn't
actually manifest itself in any performance overhead.